The alternative is to change all the places that assume phys_ram_base +
PA which I don't like very much.

We would ideally like to do this for Xen, at least in the places we
care about. (Xen uses less of the qemu tree than KVM, I think.)

Support for the map cache in the Xen tree is a rather big change that
I'm not going to attempt to support it in this patch series.

I'd rather preserve the phys_ram_base + PA assumption because it allows
us to be able to do support > 1 page DMA operations for our virtual IO
drivers. If you break the assumption that physically contiguous memory
in the guest is virtual contiguous memory in the host, things get pretty
ugly.